Early 17th century (in an earlier sense). From post-classical Latin primogenitusprimogenit + -ive. With use as noun compare primogeniture. With use as adjective compare earlier primogenital, primogenitary.
Definition of primogenitive in US English:
primogenitive
adjectiveˌprʌɪmə(ʊ)ˈdʒɛnɪtɪv
Primogenital.
Origin
Early 17th century (in an earlier sense). From post-classical Latin primogenitus primogenit + -ive. With use as noun compare primogeniture. With use as adjective compare earlier primogenital, primogenitary.